The apparatus and method of the present invention relates generally to the field of magnetostrictive testing. More particularly, the present invention concerns means for determining the magnetostrictive characteristics of a test specimen and, specifically, such means wherein the magnetostrictive characteristics are displayed through the agency of a digital output device.
Magnetostriction, the phenomenon of elastic deformation that accompanies magnetization, is a characteristic of ferromagnetic materials frequently monitored in association with the production of various types of steel products. Although quality control is the primary objective for magnetostriction testing, a secondary objective is to relate the magnetostrictive characteristics of the material being tested with its potential sound-producing properties. For quality control, the stress level of a material and the "goodness" of the material's coating are related to the magnitude and polarity of its magnetostrictive characteristics. Therefore, it is highly desirable to sample the magnetostrictive characteristics of a production run of steel for quality control purposes. In addition, knowledge of the magnetostrictive characteristics of a material would be useful in applications such as the fabrication of transformer cores where the vibration of the material, under the influence of changing magnetic fields, contributes significantly to the sound-producing properties of the transformer.
For these and various other reasons numerous devices have been developed to provide a facility whereby the magnetostrictive characteristics of materials may be measured. In this regard, prior art devices generally utilize one or more of three basic techniques in order to measure the magnetostriction of a test specimen. The three basic techniques, all of which are concerned with measuring the change in length of a test specimen subjected to a magnetic field, include the use of arrangements of mechanical levers, the use of optical apparatus and the use of electrical systems. For example, U.S. Pat. No. 2,312,888 discloses a magnetostriction measuring apparatus utilizing both mechanical levers and an optical system to sense changes of length in a test specimen subjected to a magnetic field. U.S. Pat. No. 2,596,752 discloses another magnetostriction measuring apparatus utilizing optical methods.
Apparently, the most prevalent prior art technique for performing magnetostrictive testing involves the use of electrical or electronic circuits. In prior art devices of this type, an oscilloscope or the like is frequently used as the output device for providing a display reflecting the magnetostrictive properties of a test sample which is magnetized to a particular induction, commonly of the sinusoidal variety. The oscilloscope display may take the form of a Lissajous figure wherein induction is displayed on the oscilloscope horizontal axis and the resulting specimen length variations on its vertical axis. The value of magnetostriction is then read as the vertical distance measured between the points on the display where the induction is zero and a maximum value. The latter method of reading the magnetostriction signal is commonly referred to as the "crossover to tip" method. Although other methods for reading the magnetostriction signal are known, the "crossover to tip" method has been generally preferred.
Disadvantages associated with known prior art magnetostriction testing apparatus and methods result, at least partially, from the use of analogue output display devices such as oscilloscopes. The necessity of interpreting oscilloscope displays generally requires that training programs be established for instructing equipment operators in the use of the test apparatus as well as in the reading of the displays. However, even with such training programs, reading errors are not uncommon and precise measurements are quite difficult to achieve. In addition, prior art magnetostriction testing apparatus is normally susceptible to low frequency ambient vibrations at the test location which further reduces the precision of the resulting display.
Many of the foregoing disadvantages are alleviated through the use of magnetostriction testing apparatus having digital means as the agency for displaying the value of magnetostriction. Although some attempts have been made in this direction, they have not proven to be altogether satisfactory and are not adapted for implementing the "crossover to tip" method of reading magnetostriction. See, for example, U.S. Pat. Nos. 3,807,223 and 3,838,595.